Printers of the so-called ink-jet system in which liquid or melted solid ink is discharged through nozzles, slits, porous films or the like to make recordings on materials to be recorded such as paper, cloth or film have various advantages of small size, low cost and silence in their operation. A number of ink-jet printers are therefore commercially available as black monochromatic printers and full color printers. The piezo ink-jet system using piezoelectric elements and the thermal ink-jet system in which ink droplets are formed by the action of thermal energy to make recordings are particularly useful among others in that high-speed printing is possible and high resolution is obtained.
In general, various characteristics are required for the ink-jet recording ink, and it is said that the following matters are particularly important.
(1) Good image quality having no image defect and no image unevenness even when ink is used for a long period of time.
(2) In the thermal ink-jet method, burning (kogation) due to changes in temperature does not occur on a heater site in which air bubbles are formed and developed at high temperatures, and stable images are obtained over a long period of time.
(3) Long-term storage is possible.
(4) Images obtained are excellent in storage stability.
(5) Images obtained are rapidly dried, and excellent in fixing property.
In order to improve the above-mentioned matters, a number of inventions have previously been made with respect to ink-jet recording ink, and they have come in practice.
As the ink-jet recording ink, aqueous dye ink is mainly used. However, it has problems in terms of waterfastness and lightfastness, and it is not necessarily sufficient with regard to storage stability of image. On the other hand, ink in which pigments are used as coloring materials can improve the waterfastness and the lightfastness, and gives high image density and does not feathere, so that it has a very good prospect. In recent years, a large number of inventions of this type have been proposed and have come in practice. For example, JP-A-56-147871 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a recording solution comprising an aqueous medium containing at least a pigment, a polymer dispersing agent and a nonionic surfactant, and U.S. Pat. Nos. 5,085,698 and 5,221,334 disclose the use of an AB or BAB type block copolymer as a dispersing agent for a pigment. Further, the use of a specific pigment, water-soluble resin and solvent in combination is described in U.S. Pat. No. 5,172,133.
However, particularly in the thermal ink-jet system, it has previously been difficult to keep the stability of ink for a long period of time, because heat is rapidly given to the ink in a printing head. That is, when the ink is rapidly heated, the dispersibility of a pigment contained therein becomes unstable to cause coagulation of pigment particles, resulting in development of large masses (aggregated masses) in the ink. Further, when this ink is used for a long period of time, there is the problem that not only the large masses thus developed bring about the kogation on a heater, but also an ink passage in a print head is clogged with the masses to block the flow of the ink. Furthermore, printing on paper by use of such ink not only prolongs the time required for drying, but also significantly deteriorates fixing on paper, because these large masses can not penetrate into paper fibers.